Drill bit lubricant utilizing a sulfur-phosphorous EP agent

ABSTRACT

A lubricant is shown and described. The lubricant includes about 1 to 15 weight percent of a sulfur-phosphorus extreme pressure agent, about 5 to 40 weight percent of a thickener, preferably a metal-complex soap, and a basestock. The basestock may be a mineral oil, a synthetic oil, or a combination thereof. Lubricant additives such as corrosion inhibitors, oxidation inhibitors, and anti-wear agents may also be added.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to a lubricant for lubricating journalbearings in a rock bit for drilling earth formation.

2. Background Art

Rock bits are employed for drilling wells in subterranean formations.Such bits have a body connected to a drill string and a single rollercone or a plurality (typically two or three) of roller cones mounted onthe body for drilling rock formations. The roller cones are mounted onjournals or pins integral with the bit body at its lower end. In use,the drill string and bit body are rotated in the bore hole, and eachcone rotates on its respective journal as the cone contacts the bottomof the bore hole being drilled.

Drill bits are used in hard, often tough formations and, therefore, highpressures and temperatures are encountered. The total useful life of adrill bit is typically on the order of 20 to 200 hours for bits in sizesof about 6 to 28 inch diameter at depths of about 5,000 to 20,000 feet.Useful lifetimes of about 65 to 150 hours are typical. When a drill bitwears out or fails as a bore hole is being drilled, it is necessary towithdraw the drill string to replace the bit which is a very expensiveand time consuming process. Prolonging the lives of drill bits minimizesthe lost time in “round tripping” the drill string for replacing bits.

Replacement of a drill bit can be required for a number of reasons,including wearing out or breakage of the structure contacting the rockformation. One reason for replacing the rock bits includes failure orwear of the journal bearings on which the roller cones are mounted. Thejournal bearings are subjected to very high drilling loads, highhydrostatic pressures in the hole being drilled, and high temperaturesdue to drilling, as well as elevated temperatures in the formation beingdrilled. The operating temperature of the grease in the drill bit canexceed 300° F. Considerable work has been conducted over the years toproduce bearing structures and employ materials that minimize wear andfailure of such bearings.

A variety of grease compositions have been previously employed. U.S.Pat. No. 4,358,384 discloses one prior art grease composition thatconsists of a petroleum derived mineral oil lubricant basestock and ametal soap or metal complex soap including aluminum, barium, calcium,lithium, sodium or strontium metals. A lighter, lower-viscositybasestock is generally employed to obtain low temperature greases, and aheavier, higher-viscosity basestock is used to obtain high temperaturegreases.

In order to enhance the lubricating capacity of typical petroleumbasestock greases, solid additives such as molybdenum disulfide, copper,lead or graphite have been previously added. Synthetic polymer extremepressure (EP) agents and high viscosity synthetic polymers may also beused. These materials serve to enhance the ability of the lubricant basestock to form a friction-reducing film between the moving metal surfacesunder conditions of extreme pressure and to increase the load carryingcapacity of the lubricants. The function of the lubricant is to minimizewear and to prevent scuffing and welding between contacting surfaces.U.S. Pat. Nos. 4,358,384, 3,062,741, 3,107,878, 3,281,355, and 3,384,582disclose the use of molybdenum disulfide, and other solid additives suchas copper, lead and graphite, which have been employed to attempt toenhance the lubrication properties of oils and greases.

Without being restricted to any method, in drilling applications, themechanism of lubrication is by way of hydrodynamic lubrication. When atrest, the journal and the journal bearings of a drill bit squeeze outthe lubricant and make direct contact. As the journal begins to rotate,the lubricant is drawn into the space between contacting surfaces toform a fluid wedge there between. As the journal rotation increasesspeed, this fluid wedge pushes the journal off the bearings and forms alubricating film between the contacting surfaces. The film thickness isdetermined by both the rotation speed and load capacity of thelubricant. If a film is too thin, the asperities may make contact with agreater force, resulting in shearing action between the surfaces insteadof a sliding action, which in turn generates heat and wears down thecontacting surfaces. When this happens, EP additives in the lubricantare activated by the high temperature resulting from the extremepressure to react with the exposed metal surfaces and form a protectivecoating thereon.

However, the use of solid EP agents, which improve the load carryingcapacity of a lubricant, has been shown to contribute to excessive sealand gland wear and drill bit seal failure. For example, drill bitlubricant compounds comprising a copper EP agent have displayed sealfailure due to copper deposits and loading near the seal area. Thecopper accumulates near the seal area until the seal is abraded by theconstant and progressive erosive contact with the copper deposit. Theabraded seal eventually loses its capacity to retain the greasecomposition in the journal area, permitting metal to metal contactbetween the roller cone and the journal, causing drill bit failure.Conversely, lubricants that reduce seal and gland wear typically lacksufficient film strength, that is, load carrying capacity, to be used asa drill bit lubricant.

Additionally, the use of solid EP agents comprising heavy metalcomplexes is not desirable due to their general toxicity andenvironmental impact for risks of leaking or spilling of heavymetal-containing lubricant during use, storage or disposal of thelubricant.

Accordingly, there exists a need for a lubricant that exhibits both atight seal and good load carrying capacity with reduced seal and glandwear.

SUMMARY OF THE INVENTION

In one aspect, embodiments of the present invention relate to alubricant for a drill bit, that includes from about 1 to about 15 weightpercent of a sulfur-phosphorus EP agent, wherein the sulfur-phosphorusEP agent comprises at least one selected from a substituted1,3,4-thiadiazole, a sulfunized oxymolybdenum organo phosphorodithioate,sulfur-phosphorus ester amine salts, a zinc dithiophosphate, andcombinations thereof, from about 5 to about 40 weight percent of athickener, and a basestock.

In another aspect, embodiments of the present invention relate to aroller cone drill bit including a bit body, at least one leg extendingdownward from the bit body, wherein each leg has a journal and eachjournal has a bearing surface, a roller cone mounted on each journal,wherein each roller cone has a bearing surface, a grease reservoir incommunication with the bearing surfaces; and a lubricating compositionin the grease reservoir and adjacent the bearing surfaces, where thelubricating composition includes from about 1 to about 15 weight percentof a sulfur-phosphorus EP agent, wherein the sulfur-phosphorus EP agentincludes at least one selected from a substituted 1,3,4-thiadiazole, asulfunized oxymolybdenum organo phosphorodithioate, sulfur-phosphorusester amine salts, a zinc dithiophosphate, and combinations thereof,from about 5 to about 40 percent lubricant weight of a thickener, and abasestock.

In another embodiment, the present invention relates to a method forlubricating a roller cone drill bit. The method includes the steps ofproviding a roller cone drill bit having a bit body, a grease reservoir,and at least one roller cone mounted on the bit body with at least onerotatable journal bearing; and filling the grease reservoir with alubricant, where the lubricant includes from about 1 to about 15 weightpercent of a sulfur-phosphorus EP agent, wherein the sulfur-phosphorusEP agent comprises at least one selected from a substituted1,3,4-thiadiazole, a sulfunized oxymolybdenum organo phosphorodithioate,sulfur-phosphorus ester amine salts, a zinc dithiophosphate, andcombinations thereof; from about 5 to about 40 percent lubricant weightof a thickener, and a basestock.

In yet another embodiment, the present invention relates to a method fordrilling through an earth formation. The method includes the steps ofproviding a roller cone drill bit having a bit body, a grease reservoir,and at least one roller cone mounted on the bit body with at least onerotatable journal bearing, wherein the grease reservoir contains alubricant that includes from about 1 to about 15 weight percent of asulfur-phosphorus EP agent, wherein the sulfur-phosphorus EP agentcomprises at least one selected from a substituted 1,3,4-thiadiazole, asulfunized oxymolybdenum organo phosphorodithioate, sulfur-phosphorusester amine salts, a zinc dithiophosphate, and combinations thereof,from about 5 to about 40 percent lubricant weight of a thickener, and abasestock; securing the drill bit to the end of a drill string; androtating the drill bit under an applied load on the earth formation.

Other aspects and advantages of the invention will be apparent from thefollowing description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a semi-schematic perspective of a rock bit lubricated with alubricant according to the present invention;

FIG. 2 is a partial cross-section of the drill bit in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

In one aspect, embodiments of the present invention relate to lubricantshaving a sulfur-phosphorus EP agent. In particular, embodiments of theinvention relate to lubricants for drill bits, methods for lubricating,drill bits comprising the lubricants, and methods for drilling.

A lubricant according to the present invention comprises asulfur-phosphorus EP agent, a basestock, and one or more thickeners.Additionally, additives such as anti-wear agents, corrosion inhibitors,wetting agent, and antioxidants may also be added. Details of eachcomponent are further explained below:

Sulfur-Phosphorus EP Agents:

Applications under extreme pressure conditions often rely on additives.Lubricants containing additives that protect against extreme pressureare called EP lubricants. Common EP compounds include compounds ofboron, phosphorus, sulfur, and chlorine. The compounds are typicallyactivated by the high temperature that results from the extremepressure. It has been shown that the activated compounds react withexposed metal surfaces and form a protected coating which covers exposedasperities.

In one embodiment, a sulfur-phosphorous EP agent according to thepresent invention includes at least one selected from a substituted1,3,4-thiadiazole, a sulfunized oxymolybdenum organo phosphorodithioate,phosphorous-sulfur ester amine salts, a zinc dithiophosphate, orcombinations thereof. In another embodiment, a sulfur-phosphorus EPagent according to the present invention includes a blend of asubstituted 1,3,4-thiadiazole and a sulfunized oxymolybdenum organophosphorodithioate. Specifically, when making a lubricant of the presentinvention, the lubricant may include from about 1 to about 5 weightpercent of the substituted 1,3,4-thiadiazole and from about 1 to about10 weight percent of the sulfunized oxymolybdenum organophosphorodithioate. Preferably, about 3 weight percent of thesubstituted 1,3,4-thiadiazole and about 4 weight percent of thesulfunized oxymolybdenum organo phosphorodithioate are added to thelubricant. One example of 1,3,4-thiadiazole is1,3,4-thiadiazole-2(3H)-thione,5,5-dithiobis. One example of sulfunizedoxymolybdenum organo phosphorodithioates is molybdenum di(2-ethylhexyl)phosphorodithioate. One example of a phosphorous-sulfur ester amine saltincludes Lubrizol™ 5080A, commercially available from The LubrizolCorporation (Wickliffe, Ohio). Examples of zinc dithiophosphates includeLubrizol™ 677A, Lubrizol™ 139, and Lubrizol™ 5002, all commerciallyavailable from The Lubrizol Corporation (Wickliffe, Ohio).

In other embodiments, a sulfur-phosphorus EP agent may be added as anadditive to a commercially available grease composition. Advantageously,sulfur-phosphorus EP agents, in accordance with embodiments of thepresent invention, may be used as an additive in a wide range ofbasestocks and greases, such as Molykote™ G-0101, available from DowCorning (Midland, Mich.), Mobilith™ AW-2, available from ExxonMobilCorporation (Fairfax, Va.), and Lucant 600 and Lucant 2000, bothavailable from Mitsui Petrochemical (New York, N.Y.).

Basestocks:

The basestock, or base oil, form the main lubricating component. Oilsare generally classified as refined and synthetic. Refined oils are alsoreferred to as mineral oils or petroleum oils. For example, paraphinicand naphthenic are refined from crude oil while synthetic oils aremanufactured by chemical synthesis. The basestock may be selected fromany of the basestocks known in the art, including a synthetic base oil,a petroleum or mineral oil, or combinations thereof. In someembodiments, a synthetic lubricant basestock may be preferred over apetroleum derived basestock to increase viscosity. In other embodiments,a high viscosity petroleum derived mineral oil basestock may be used.

Suitable synthetic oils for use in a basestock may include syntheticpolyalphaolefins, other hydrocarbon fluids and oils, syntheticpolyethers, poly-esters, alkylene oxide polymers, and interpolymers,esters of phosphorus containing acids, silicon based oils and mixturesthereof. In one embodiment, the basestock may include a high viscosityindex polyalphaolefin based fluid. Suitable polyalphaolefins includethose discussed in U.S. Pat. Nos. 5,589,443, 5,668,092, and 4,827,064,which are incorporated herein by reference in their entirety. Othersuitable synthetic oils include alkylated naphthalenes, such asSynesstic™ AN, which is available from ExxonMobil Corporation (Fairfax,Va.), polybutenes, such as Indopol™ polybutenes which are available fromBP P.L.C. (Warrenville, Ill.), and hydrogenated polybutenes, such asPanalane™ hydrogenated polybutenes, which are available from BP P.L.C.(Warrenville, Ill.).

Suitable mineral or petroleum oils may include naphthenic or paraffinicoil. Other suitable mineral oils may include high viscosity indexhydroprocessed basestock and bio-based esters.

Thickeners:

Thickeners give a lubricant its characteristic consistency and aresometimes thought of as a “three-dimensional fibrous network” or“sponge” that holds the oil in place.

In one embodiment, the base oil may be thickened with a soap, such assoaps of calcium, aluminum, titanium, barium, lithium, and theircomplexes. Metal complex soaps may include alkali metals, alkaline earthmetals, Group IVB metals, and aluminum. Simple soaps may be formed bycombining a fatty acid or ester with a metal and reacting through asaponification process, with the application of heat, pressure, oragitation. While simple soaps are formed by reacting one single organicacid with a metal hydroxide, complex soaps may be formed by reacting twoor more organic compounds with the metal hydroxide.

In another embodiment, the base oil may be thickened with a non-soap,such as urea, fine silica, fine clay, and/or silica gel. In yet anotherembodiment, the basestock may be thickened with both soap and non-soapthickening agents. While the above description lists several specificthickening agents, no limitation is intended on the scope of theinvention by such a description. It is specifically within the scope ofthe present invention that other soap and non-soap thickening agents maybe used.

Other Additives:

Additives that are commonly added to lubricants to improve theirperformances may also be added to a lubricant of the present invention.Examples of such additives are anti-wear agents, corrosion inhibitors,anti-oxidants, zinc- and chlorine-based EP agents, etc, including copperpowder molybdenum disulfide, and bismuth ethylhexanoate. Examples ofzinc- and chlorine-based EP agents include Lubrizol™ 885 and Lubrizol™2501, which are both commercially available from The LubrizolCorporation (Wickliffe, Ohio). Bismuth ethylhexanoate is commerciallyavailable from OMG Americas, Inc. (Cleveland, Ohio).

For an excellent review of common lubricant additives, see LubricantAdditives: Chemistry and Applications, edited by Leslie R. Rudnick(2003, ISBN 0824708571).

Application of the Lubricant in a Drill Bit:

Referring now FIGS. 1 and 2, a sealed bearing rotary cone rock bit,generally designated as 10, consists of bit body 12 forming an upper pinend 14 and a cutter end of roller cones 16 that are supported by legs 13extending from body 12. The threaded pin end 14 is adapted for assemblyonto a drill string (not shown) for drilling oil wells or the like. Eachof the legs 13 terminate in a shirttail portion 22. Each of the rollercones 16 typically have a plurality of cutting elements 17 pressedwithin holes formed in the surfaces of the cones for bearing on the rockformation to be drilled. Nozzles 20 in the bit body 12 introducedrilling mud into the space around the roller cones 16 for cooling andcarrying away formation chips drilled by the drill bit. While referenceis made to an insert-type bit, the scope of the present invention shouldnot be limited by any particular cutting structure. Embodiments of thepresent invention generally apply to any rock bit (whether roller cone,disc, etc.) that requires lubrication by grease.

Each roller cone 16 is in the form of a hollow, frustoconical steel bodyhaving cutting elements 17 pressed into holes on the external surface.For long life, the cutting elements may be tungsten carbide insertstipped with a polycrystalline diamond layer. Such tungsten carbideinserts provide the drilling action by engaging a subterranean rockformation as the rock bit is rotated. Some types of bits have hardfacedsteel teeth milled on the outside of the cone instead of carbideinserts.

Each leg 13 includes a journal 24 extending downwardly and radiallyinward on the rock bit body. The journal 24 includes a cylindricalbearing surface 25 which may have a flush hardmetal deposit 62 on alower potion of the journal 24.

The cavity in the cone 16 contains a cylindrical bearing surface 26. Afloating bearing 45 may be disposed between the cone and the journal.Alternatively, the cone may include a bearing deposit in a groove in thecone (not shown separately). The floating bearing 45 engages thehardmetal deposit 62 on the leg and provides the main bearing surfacefor the cone on the bit body. The end surface 33 of the journal 24carries the principal thrust loads of the cone 16 on the journal 24.Other types of bits, particularly for higher rotational speedapplications, may have roller bearings instead of the exemplary journalbearings illustrated herein.

A plurality of bearing balls 28 are fitted into complementary ball races29, 32 in the cone 16 and on the journal 24. These balls 28 are insertedthrough a ball passage 42, which extends through the journal 24 betweenthe bearing races and the exterior of the drill bit. A cone 16 is firstfitted on the journal 24, and then the bearing balls 28 are insertedthrough the ball passage 42. The balls 28 carry any thrust loads tendingto remove the cone 16 from the journal 24 and thereby retain the cone 16on the journal 24. The balls 28 are retained in the races by a ballretainer 64 inserted through the ball passage 42 after the balls are inplace. A plug 44 is then welded into the end of the ball passage 42 tokeep the ball retainer 64 in place.

Contained within bit body 12 is a grease reservoir system generallydesignated as 18. Lubricant passages 21 and 42 are provided from thereservoir to bearing surfaces 25, 26 formed between a journal bearing 24and each of the cones 16. Drilling fluid is directed within the hollowpin end 14 of the bit 10 to an interior plenum chamber 11 formed by thebit body 12. The fluid is then directed out of the bit through the oneor more nozzles 20.

The bearing surfaces between the journal 24 and cone 16 are lubricatedby a lubricant or grease composition. Preferably, the interior of thedrill bit is evacuated, and lubricant or grease is introduced through afill passage 46. The lubricant or grease thus fills the regions adjacentthe bearing surfaces plus various passages and a grease reservoir. Thegrease reservoir comprises a chamber 19 in the bit body 10, which isconnected to the ball passage 42 by a lubricant passage 21. Lubricant orgrease also fills the portion of the ball passage 42 adjacent the ballretainer. Lubricant or grease is retained in the bearing structure by aresilient seal 50 between the cone 16 and journal 24

Lubricant contained within chamber 19 of the reservoir is directedthrough lube passage 21 formed within leg 13. A smaller concentricspindle or pilot bearing 31 extends from end 33 of the journal bearing24 and is retained within a complimentary bearing formed within thecone. A seal generally designated as 50 is positioned within a sealgland formed between the journal 24 and the cone 16.

In one embodiment, a lubricant in accordance with this aspect of theinvention may include from about 1 to about 15 weight percent of asulfur-phosphorus EP agent; about 1 to about 10 weight percent ofsilica; about 5 to about 40 weight percent of a thickening agent,preferably a metal-complex soap, and a balance of a heavy mineralbasestock. In another embodiment, the lubricant may further comprisesolid additives. In one embodiment, the solid additives are from about 1to about 15 weight percent of molybdenum disulfide and from about 1 toabout 10 weight percent of copper particles. In yet another embodiment,the basestock may be a blend of mineral oil and synthetic oil.Specifically, in one embodiment, the basestock may be a blend of 0 to100% mineral oil and 100 to 0% synthetic oil with any percentagetherebetween, preferably about 50% of each.

In one embodiment, the lubricant or grease in the grease reservoir mayinclude from about 1 to about 15 weight percent of a sulfur-phosphorusEP agent; about 1 to about 10 weight percent of silica; about 5 to about40 weight percent of a thickening agent, preferably a metal-complexsoap; and a balance of a heavy mineral basestock. In another embodiment,the lubricant or grease may further comprise solid additives such asmolybdenum disulfide and copper particles. In one embodiment, the solidadditives are from about 1 to about 15 weight percent of molybdenumdisulfide and from about 1 to about 10 weight percent of copperparticles. In yet another embodiment, the basestock may be a blend of 0to 100% mineral oil and 100 to 0% synthetic oil with any percentagetherebetween, preferably about 50% of each.

Use of the Lubricant in a Method of Drilling:

According to one aspect of the present invention, a method for drillingis provided. In one embodiment, the method for drilling includes thesteps of providing a roller cone drill bit having a bit body and aplurality of roller cones mount on the bit body with rotatable journalbearings, introducing a lubricating composition to the journal bearings,where the lubricating composition includes a basestock, a thickener, anda sulfur-phosphorus EP agent. In one embodiment, the lubricant in thegrease reservoir may include from about 1 to about 15 weight percent ofa sulfur-phosphorus EP agent; about 1 to about 10 weight percent ofsilica; about 5 to about 40 weight percent of a thickening agent,preferably a metal-complex soap; and a balance of a heavy mineralbasestock. In another embodiment, the lubricant may further comprisesolid additives such as molybdenum disulfide and copper particles. Inone embodiment, the solid additives are from about 1 to about 15 weightpercent of molybdenum disulfide and from about 1 to about 10 weightpercent of copper particles. In yet another embodiment, the basestockmay be a blend of 0 to 100% mineral oil and 100 to 0% synthetic oil withany percentage therebetween, preferably about 50% of each.

In the above embodiments, if the composition of the basestock ispredominantly synthetic oil, an ester-based swelling agent may also beadded to enhance the wetting and suspension of silica. One suitableswelling agent includes Esterex C4461, which is available fromExxonMobil Corporation (Fairfax, Va.).

Lubricants in accordance with embodiments of the invention have beenfound to have superior properties, as compared to prior art lubricantsand as evidenced by the following examples.

EXAMPLES Example 1 A Sulfur-Phosphorus EP Lubricant Containing Cu andMoS₂

A lubricant in accordance with one embodiment of the present inventionwas prepared with about 5 weight percent of silica, about 12 weightpercent of molybdenum disulfide, about 32 weight percent of a Li-complexsoap, about 5 weight percent of copper particles, and about 10 weightpercent of a sulfur-phosphorus EP agent, and a balance of a basestockcontaining 100% mineral oil. The sulfur-phosphorus EP agent used in thisexample is a blend of about 3 parts of a substituted 1,3,4-thiadiazole(e.g. 1,3,4-thiadiazole,5,5-dithiobis available from R.T. VanderbiltCompany, Inc. under the trade name Vanlube® 829) and about 4 parts of asulfunized oxymolybdenum organo phosphorodithioate (e.g. molybdenumdi(2-ethylhexyl)phosphorodithioate available from R.T. VanderbiltCompany, Inc. under the trade name Molyvan® L). Such lubricant has afilm strength of 1000 kg as determined by ASTM-2596 4 ball load EP test.The benefits of using only mineral oil may include causing seal swell,which allows for a tighter seal and wetting silica gel for easiersuspension.

Example 2 A Sulfur-Phosphorus EP Lubricant Without Solid Additives

A lubricant in accordance with another embodiment of the presentinvention was prepared with about 5% of silica, about 32% of aLi-complex soap, and about 10% of the sulfur-phosphorus EP agent and abalance of a basestock containing 100% mineral oil. Thesulfur-phosphorus EP agent used in this example is a blend of about 3parts of a substituted 1,3,4-thiadiazole (e.g.1,3,4-thiadiazole,5,5-dithiobis available from R.T. Vanderbilt Company,Inc. under the trade name Vanlube® 829) and about 4 parts of asulfunized oxymolybdenum organo phosphorodithioate (e.g. molybdenumdi(2-ethylhexyl)phosphorodithioate available from R.T. VanderbiltCompany, Inc. under the trade name Molyvan® L). Such lubricant has afilm strength of 1000 kg as determined by ASTM-2596 4 ball load EP test.No solid particles such as copper or molybdenum disulfide were added.

Example 3 A Sulfur-Phosphorus EP Lubricant with Synthetic Basestock

A lubricant in accordance with yet another embodiment of the presentinvention was prepared with about 10% of a sulfur-phosphorus EP agent,about 5% of silica, about 32% of a Li complex-metal soap, and a balanceof a one-to-one blend of synthetic oil and mineral oil as the basestock.The sulfur-phosphorus EP agent used in this example is a blend of about3 parts of a substituted 1,3,4-thiadiazole (e.g.1,3,4-thiadiazole,5,5-dithiobis available from R.T. Vanderbilt Company,Inc. under the trade name Vanlube® 829) and about 4 parts of asulfunized oxymolybdenum organo phosphorodithioate (e.g. molybdenumdi(2-ethylhexyl)phosphorodithioate available from R.T. VanderbiltCompany, Inc. under the trade name Molyvan® L). No solid particles suchas copper or molybdenum disulfide were added. The addition of syntheticoil may afford such a lubricant better temperature characteristics, andthe lack of solids may render the lubricant more compatible withmechanical seal systems. To enhance wetting and suspension of the silicagel thickening agent, an ester-based swelling agents was also added.

Example 4 A Sulfur-Phosphorus EP Lubricant Containing Other Additives

A lubricant according to yet another embodiment of the present inventionwas prepared with about 10% of the sulfur-phosphorus EP, about 32% of aLi-complex soap, about 3% of a corrosion inhibitor (such as, forexample, Hi Tech 350 available from Hi Tech Company), and a balance of abasestock. The sulfur-phosphorus EP agent used in this example containsabout 3 parts of a substituted 1,3,4-thiadiazole (e.g.1,3,4-thiadiazole,5,5-dithiobis available from R.T. Vanderbilt Company,Inc. under the trade name Vanlube® 829) and about 4 parts of asulfunized oxymolybdenum organo phosphorodithioate (e.g. molybdenumdi(2-ethylhexyl)phosphorodithioate available from R.T. VanderbiltCompany, Inc. under the trade name Molyvan® L). The basestock is a blendof a mineral oil and ultra-high molecular weight polyalphaolefin (suchas Lucant 2000 and Lucant 600 available from Mitsui Petrochemical, NewYork, N.Y.) in one-to-one ratio. About 6% of molybdenum disulfide andabout 1.5% of silica were also added. Such composition may have theadvantage of avoiding vulcanization of seals that are made of HSNelastomer material.

Advantages of the embodiments of the invention may include one or moreof the following: adding a sulfur-phosphorus EP agent to make alubricant according to the present invention may eliminate the need toadd solid EP additives such as lead. A lubricant thus produced may benon-toxic, environmentally friendly and still retain exceptional loadcarrying ability required in applications such as lubrication of rockbits. In addition, by combining a high to medium viscosity syntheticbase oil with an almost equal amount of mineral oil, a lubricant thathas efficient, controlled seal swell may be produced to ensure themaintenance of a tight seal during the drilling process withoutexcessive seal wear.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

1. A lubricant for a drill bit, comprising: from about 1 to about 15weight percent of a sulfur-phosphorus EP agent, wherein thesulfur-phosphorus EP agent comprises at least one selected from asubstituted 1,3,4-thiadiazole, a sulfunized oxymolybdenum organophosphorodithioate, sulfur-phosphorus ester amine salts, a zincdithiophosphate, and combinations thereof. from about 5 to about 40weight percent of a thickener; and a basestock.
 2. The lubricant ofclaim 1, wherein the sulfur-phosphorus EP agent comprises: about 1 toabout 5 weight percent of 1,3,4-thiadiazole-2(3H)-thione,5,5-dithiobis;and about 1 to about 10 weight percent of molybdenumdi(2-ethylhexyl)phosphorodithioate;
 3. The lubricant of claim 1, furthercomprising: at least one additive selected from the group consisting ofcorrosion inhibitors, oxidation inhibitors, anti-wear agents, and zinc-and chlorine-based EP agents.
 4. The lubricant of claim 1, wherein thethickening agent is selected from at least one from the group consistingof a soap and a non-soap, wherein the soap includes soaps of calcium,aluminum, titanium, barium, lithium, and complexes thereof, and thenon-soap includes urea, fine silica, fine clay, and silica gel.
 5. Thelubricant of claim 1, wherein the combination of basestock comprisesfrom 0 to 100% mineral oil and 100 to 0% synthetic oil, or anypercentage therebetween.
 6. The lubricant of claim 1, further comprisingfrom about 1 to about 10 weight percent of silica, about 5 to about 40percent of a metal-complex soap by weight, and wherein the basestockcomprises from 100 to 0% mineral oil and 0 to 100% synthetic oil, or anypercentage therebetween.
 7. The lubricant of claim 6, further comprisesfrom about 1 to about 10 weight percent of copper particles, about 1 toabout 15 of molybdenum disulfide.
 8. The lubricant of claim 7, furthercomprising an ester-based swelling agent.
 9. A roller cone drill bit,comprising: a bit body; at least one leg extending downward from the bitbody, wherein each leg has a journal and each journal has a bearingsurface; a roller cone mounted on each journal, wherein each roller conehas a bearing surface; a grease reservoir in communication with thebearing surfaces; and a lubricating composition in the grease reservoirand adjacent the bearing surfaces, the lubricating compositioncomprising: from about 1 to about 15 weight percent of asulfur-phosphorus EP agent, wherein the sulfur-phosphorus EP agentcomprises at least one selected from a substituted 1,3,4-thiadiazole, asulfunized oxymolybdenum organo phosphorodithioate, sulfur-phosphorusester amine salts, a zinc dithiophosphate, and combinations thereof;from about 5 to about 40 percent lubricant weight of a thickener; and abasestock.
 10. The roller cone drill bit of claim 9, wherein thebasestock comprises from 0 to 100% mineral oil and 100 to 0% syntheticoil, or any percentage therebetween.
 11. The roller cone drill bit ofclaim 9, further comprising: an additive comprising at least one elementselected from the group consisting of corrosion inhibitors, oxidationinhibitors, anti-wear agents, and zinc- and chlorine-based EP agents.12. The roller cone drill bit of claim 9, wherein the thickener is atleast one selected from the group consisting of a soap and a non-soap,wherein the soap includes soaps of calcium, aluminum, titanium, barium,lithium, and complexes thereof, and the non-soap includes urea, finesilica, fine clay, and silica gel.
 13. A method for lubricating a rollercone drill bit, comprising: providing a roller cone drill bit having abit body, a grease reservoir, and at least one roller cone mounted onthe bit body with at least one rotatable journal bearing; and fillingthe grease reservoir with a lubricant, the lubricant comprising: fromabout 1 to about 15 weight percent of a sulfur-phosphorus EP agent,wherein the sulfur-phosphorus EP agent comprises at least one selectedfrom a substituted 1,3,4-thiadiazole, a sulfunized oxymolybdenum organophosphorodithioate, sulfur-phosphorus ester amine salts, a zincdithiophosphate, and combinations thereof; from about 5 to about 40percent lubricant weight of a thickener; and a basestock.
 14. The methodof claim 13, further comprising: contacting the at least one journalbearing with the lubricant.
 15. The method of claim 13, wherein thebasestock comprises from 0 to 100% mineral oil and 100 to 0% syntheticoil, or any percentage therebetween.
 16. The method of claim 13, whereinthe lubricant further comprises an additive comprising at least oneadditive selected from the group consisting of oxidation inhibitors,corrosion inhibitors, anti-wear agents, and zinc- and chlorine-based EPagents.
 17. A method for drilling through an earth formation,comprising: providing a roller cone drill bit having a bit body, agrease reservoir, and at least one roller cone mounted on the bit bodywith at least one rotatable journal bearing, wherein the greasereservoir contains a lubricant, the lubricant comprising: from about 1to about 15 weight percent of a sulfur-phosphorus EP agent, wherein thesulfur-phosphorus EP agent comprises at least one selected from asubstituted 1,3,4-thiadiazole, a sulfunized oxymolybdenum organophosphorodithioate, sulfur-phosphorus ester amine salts, a zincdithiophosphate, and combinations thereof; from about 5 to about 40percent lubricant weight of a thickener; and a basestock; securing thedrill bit to the end of a drill string; and rotating the drill bit underan applied load on the earth formation.
 18. The method of claim 17,further comprising: introducing the lubricating composition to thejournal bearings.
 19. The method of claim 17, wherein the basestockcomprises from 0 to 100% mineral oil and 100 to 0% synthetic oil, or anypercentage therebetween.
 20. The method of claim 17, wherein thelubricating composition further comprises an additive comprising atleast one additive selected from the group consisting of oxidationinhibitors, corrosion inhibitors, anti-wear agents, and zinc- andchlorine-based EP agents.